There are many applications in which multiple images must be displayed either sequentially or simultaneously. Current apparatus which can project multiple images are bulky, usually being complete duplicates of single-image projection apparatus. Typically, multiple light sources are required.
However, there is a need in the art for compact projection apparatus which can project multiple images, in which the multiple images each optionally display different image information. Such a projector could be used for wide-screen projection, three-dimensional image creation, and interactive imaging applications.
Polarization beam splitters for such projection apparatus must also be compact. In addition to being compact, the PBS structures must be easy to fabricate since machining of small optical components greatly increases their expense. Thus there is a need in the art for improved polarization beam splitters which are compact and easy to fabricate. Such polarization beam splitters can be used in compact image projection apparatus.
FIGS. 2A-2C depict polarization beam splitters with two LCoS spatial light modulators. The polarization beam splitter needs to have the same vertical and horizontal lengths. However, light has a certain cone angle; the longer the length, the bigger the cone will be. As seen in FIG. 2B, there is significant light leakage from the polarization beam splitter. This leakage will cause an undesirable ghost image due to total internal reflection. Thus it is necessary, for this design, to either enlarge the size of the PBS or control the light solid angle. Further, as seen in FIG. 2C, there are large regions (indicated in part by the oval shapes) of the PBS that are unused by the light emitted from the spatial light modulator, making the device inefficient.
As seen in FIG. 3, a more efficient polarization beam splitter with less light leakage and greater PBS volume utilization can be created using stepped optical surfaces adjacent to the spatial light modulators (as well as stepped surfaces on the top and bottom PBS surfaces). While such a design advantageously reduces size, improves brightness, and improves contrast, fabrication is difficult, particularly for the precision machining of the stepped optical surfaces.
Thus, there remains a need in the art for improved polarization beam splitters that are compact, have high brightness and contrast, and are simple and cost-effective to fabricate.